In the construction of gas turbine engines, it is often necessary to create a seal between adjacent hardware components to prevent, or at least control, leakage of fluids between the components.
FIG. 1 illustrates a location in a gas turbine engine, in which a known type of rope seal arrangement 10 is used to seal the gap G between two adjacent components 12 and 14. Components 12 and 14 may be, for example, parts of a combustor, gap G being necessary to allow movement of component 12 relative to component 14 in the directions indicated by the arrows D. Such movements are due to differential thermal expansion and contraction of the components 12 and 14 and other components of the combustor, or other engine structure, to which the components are fixed. Another cause of such movements may be pressure differences or thrusts on the components due to the combustion reaction in the combustor. As will be realized by the skilled person, it is important to prevent high-pressure gases 16 from leaking excessively through the gap G past the seal 10. This is a particularly difficult task, due to the above-mentioned high temperatures, movements and pressure differentials, but rope-type seals are considered in the heavy-duty gas turbine industry to be a cost-effective means of preventing, or at least minimizing, such leakage.
However, problems arise in that rope seals tend to permanently deform under load at high temperatures and therefore, after the load is removed, do not return to their original dimensions.
To explain further, the rope seal arrangement 10 comprises a length of so-called “rope” 18 housed in a recess 20 in component 14 and of sufficient diameter so that part of its circumference projects out of the recess and stands proud of the outer surface 21 of the component. The rope 18 is typically of woven or plaited construction and comprises refractory ceramic fibers and/or metallic wires. The components 12 and 14 are assembled into the combustor so that in the cold condition the rope 18 is compressed against the surface 22 of component 12. However, when the combustor heats up in use, the components 12 and 14 move with respect to each other and the width of gap G varies, causing the rope to be either further compressed, if the gap reduces, or released somewhat from compression if the gap increases. In case of further compression, although the rope has a certain amount of inherent resilience, this is reduced at high temperatures and the crushing forces caused by the reduction of gap G may exceed the limit of the inherent resilience, so that the rope is liable to become permanently deformed. Hence, next time the combustor is started from cold, there may be a gap between the sealing rope 18 and the surface 22 of component 12. In case of release from compression, high pressure differences between the two sides of the seal may also cause the sealing rope to lift away from its seating in the recess of component 14. Furthermore, with a small diameter rope and/or a shallow recess, there could also be a danger of the seal being blown out of the seating and through the gap G in the direction of the lower pressure.